1. Field of the Invention
This invention relates to systems for creating images of a scene. More specifically, this invention relates to systems operative to generate a scene image spanning a wide field of regard.
While the present invention is described herein with reference to a particular embodiment, it is understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional embodiments within the scope thereof.
2. Description of the Related Art
Infrared imaging systems are used in a variety of military and commercial applications to provide either an operator or a guidance system with an infrared view of a scene. IR imaging systems are typically characterized as having a "field of regard" which refers to the angular breadth of the resultant scene image. One benefit accruing from a wide field of regard is that a viewer of the wide-field image may observe individual objects therein within the context of a larger scene. However, in conventional imaging systems increases in the field of regard generally come at the expense of decreases in image resolution. Image resolution may also be impaired by variable atmospheric conditions.
Various methods have been utilized to avoid the necessity of striking a balance between image resolution and field of regard. For example, in certain imaging systems a mirror is operative to pan across the field of regard by rotating about an axis. Rotation of the mirror allows a linear array of detectors in optical alignment therewith to collect radiant energy from across the field of regard. The radiant energy from the scene within the field of regard is focused upon the detector array by one of a pair of lenses. One of the lenses encompasses a wide field of view, while the other covers a relatively narrow field of view. The lenses are then mechanically moved in and out of the optical train of the imaging system to alternately provide a wide field of regard or improved resolution.
One disadvantage of this approach is that the rate at which an operator may switch between the fields of view of the two lenses is limited by the response of the servo system used to alternately interpose the lenses within the optical train. In addition, it is often difficult to capture a moving object within the field of view of the high resolution lens even though the location of the object may be readily apparent within the wider field of view.
In a second approach, an imaging sensor (such as the linear array described above) is mounted on a gimbal scan mechanism. The gimbal is rotated to direct the field of view of the sensor to various regions within the field of regard, with frames of image data being produced by the sensor at a known rate (e.g. 60 Hz). Although individual regions throughout the entire field of regard may be viewed in isolation using this method, a composite image of the entire field of regard is not produced. It is also generally difficult to maintain a moving object within the sensor field of view (by rotation of the gimbal) without "smearing" the resultant image. Moreover, complex processing methods are required to create images across consecutive frames of image data.
A further complication associated with a gimbal scanned sensor system is due to the fact that successive image frames must be aligned in real time in order to display a portion of the field of regard which subtends an angle exceeding that of a single image frame. This alignment is typically effected by use of a plurality of pickoff detectors to ascertain the instantaneous angular orientation of the gimbal within the field of view. The position information garnered from the pickoff detectors is used by the display driver to appropriately register successive frames on a viewing display. Unfortunately, pickoff inaccuracies and gimbal platform dynamics limit alignment of adjacent pixels. Moreover, the image registration process is impaired as a result of spurious acceleration of the gimbal. That is, changes in the angular velocity of the gimbal between pickoff points can lead to misalignment of frame images in the viewing display.
In a third approach, image data from a number of separate sensors is used to generate a real-time image of an entire field of regard. The fields of view of the individual sensors are often arranged to slightly overlap in order to prevent seams from appearing in the composite image. However, complex and expensive image processing hardware is typically required to implement this multi-sensor scheme. In addition, multi-sensor systems offer only minimal improvement in signal-to-noise ratio relative to single sensor systems.
In each of the aforementioned conventional imaging schemes the number of individual detectors included within sensors employed therein has dramatically increased in recent years. In particular, advances in semiconductor fabrication techniques have significantly increased the average yield (number of properly functioning detectors per semiconductor chip) of batch-processed detector arrays. Nonetheless, since current yields average less than 100%, a number of defective detectors are typically present within each array. An interpolation scheme wherein the data from adjacent detectors is averaged has been employed to partially compensate for these defective detectors. However, since "real" pixels are still missing from the region canvassed by the defective detector, the resultant image remains degraded even after such compensation. As a consequence, targets associated with such pixels may be overlooked or misinterpreted.
It follows that a need in the art exists for a single sensor, high resolution imaging system having a wide field of regard.